This invention is related to concurrently filed U.S. patent applications Ser. No. 737,992, filed May 28, 1985, Ser. No. 737,990, filed May 28, 1985, and Ser. No. 737,991, filed May 28, 1985.
The invention concerns a method for reducing lost circulation when aqueous or oil based drilling fluids are used. More particularly, the method involves dispersing a water absorbent and hydrocarbon repellent polymer in a hydrocarbon fluid, which will prevent the polymer from absorbing water and expanding to plug fissures and thief zones until water absorption is desired.
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clay solids used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
A variety of different substances are now pumped down well bores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Different forms of cellulose are the preferred materials employed. Some substances which have been pumped into well bores to control lost circulation are: almond hulls, walnut hulls, bagasse, dried tumbleweed, paper, coarse and fine mica, and even pieces of rubber tires. These and other prior art additives are described in U.S. Pat. No. 4,498,995.
Another process that is employed to close off large lost circulation problems is referred to in the art as gunk squeeze. In the gunk squeeze process, a quantity of a powdered bentonite is mixed in diesel oil and pumped down the well bore. Water injection follows the bentonite and diesel oil. If mixed well, the water and bentonite will harden to form a gunky semi-solid mess, which will reduce lost circulation. Problems frequently occur in trying to adequately mix the bentonite and water in the well. The bentonite must also be kept dry until it reaches the desired point in the well. This method is disclosed in U.S. Pat. No. 3,082,823.
Many of the methods devised to control lost circulation involve the use of a water expandable clay such as bentonite which may be mixed with another ingredient to form a viscous paste or cement. U.S. Pat. No. 2,890,169 discloses a lost circulation fluid made by forming a slurry of bentonite and cement in oil. The slurry is mixed with a surfactant and water to form a composition comprising a water-in-oil emulsion having bentonite and cement dispersed in the continuous oil phase. As this composition is pumped down the wellbore, the oil expands and flocculates the bentonite which, under the right conditions, forms a filter cake on the wellbore surface in the lost circulation area. Hopefully, the filter cake will break the emulsion causing the emulsified water to react with the cement to form a solid coating on the filter cake. But such a complex process can easily go wrong.
U.S. Pat. No. 3,448,800 discloses another lost circulation method wherein a water soluble polymer is slurried in a nonaqueous medium and injected into a well. An aqueous slurry of a mineral material such as barite, cement or plaster of paris is subsequently injected into the well to mix with the first slurry to form a cement-like plug in the wellbore.
U.S. Pat. No. 4,261,422 describes the use of an expandable clay such as bentonite or montmorillonite which is dispersed in a liquid hydrocarbon for injection into the well. After injection, the bentonite or montmorillonite will expand upon contact with water in the formation. Thus, it is hoped that the expanding clay will close off water producing intervals but not harm oil producing intervals.
A similar method is disclosed in U.S. Pat. No. 3,078,920 which uses a solution of polymerized methacrylate dissolved in a nonaqueous solvent such as acetic acid, acetic anhydride, propionic acid and liquid aliphatic ketones such as acetone and methyl-ethyl ketone. The methacrylate will expand upon contact with formation water in the water producing intervals of the well.
It has also been proposed to mix bentonite with water in the presence of a water soluble polymer which will flocculate and congeal the clay to form a much stronger and stiffer cementlike plug than will form if bentonite is mixed with water. U.S. Pat. No. 3,909,421 discloses such a fluid made by blending a dry powdered polyacrylamide with bentonite followed by mixing the powder blend with water. U.S. Pat. No. 4,128,528 claims a powdered bentonite/polyacrylamide thickening composition prepared by mixing a water-in-oil emulsion with bentonite to form a powdered composition which rapidly becomes a viscous stiff material when mixed with water. U.S. Pat. Nos. 4,503,170; 4,475,594; 4,445,576; 4,442,241 and 4,391,925 teach the use of a water expandable clay dispersed in the oily phase of a water-in-oil emulsion containing a surfactant to stabilize the emulsion and a polymer dispersed in the aqueous phase. When the emulsion is sheared, it breaks and a bentonite paste is formed which hardens into a cement-like plug. The patent discloses the use of such polymers as polyacrylamide, polyethylene oxide and copolymers of acrylamide and acrylic or methacrylic acid.
U.S. Pat. No. 4,124,748 discloses a cross-linked copolymer of a vinyl ester and an ethylenically unsaturated carboxylic acid or derivative thereof that can absorb 200-800% of its weight in water and expand substantially in volume when doing so. Another highly water absorbent, expanding copolymer is described in U.S. Pat. No. 4,320,040. The described compound is derived by polymerizing acrylic acid and/or methacrylic acid in the presence of polyvinyl alcohol followed by neutralization and heat treatment.